1. Field of the Invention
The present invention relates to an improved acoustic signal-processing apparatus and method operable to compensate a lack of a bass sound band to provide an increased feeling of bass sound. In particular, it relates to an improved art that is operable to add a low frequency component-related overtone to provide an increased feeling of bass sound, and that is preferred for the use of, e.g., a small-sized speaker unit or an instrument prone to a deficiency in a feeling of bass sound.
2. Description of the Related Art
In general, it is well known that a small-size speaker unit is insufficient to regenerate sound at a bass sound band. One of known methods for smoothing out the issue is to regenerate harmonic overtones based on regeneration-resistant bass sound instead of regenerating the regeneration-resistant bass sound. It is well known that, according to the method as discussed above, a virtual pitch effect provides an improved feeling of audible bass sound, although the harmonic overtones are reproduced within the range of a speaker reproducible band.
The term “harmonic overtone” has two different meanings. According to one of the two different definitions, the “harmonic overtone” refers to any sound component that excludes a fundamental tone (a sound having a fundamental frequency) in a musical tone or original sound, and that has a frequency equal to a positive integer multiple of a frequency of the fundamental tone.
According to the other definition, the “harmonic overtone” refers to a sound having a frequency equal to a positive integer multiple of a frequency of a target sound.
The “harmonic overtone” herein is not differentiated from one another as above, but is simply called an “overtone”. Furthermore, an overtone having a frequency equal to an “n”-multiple (“n” is a positive integer) of a frequency of the fundamental tone or original sound is herein referred to as an “n”-fold overtone.
The following discusses two different types of prior art acoustic signal-processing apparatuses with reference to FIGS. 9 and 10.
FIG. 9(a) is a block diagram illustrating a first prior art acoustic signal-processing apparatus. As illustrated in FIG. 9(a), a signal that has entered the first acoustic signal-processing apparatus through an input terminal 1 is diverted into two systems. In the first system, one of the diverted input signals is fed into an adder 7 through one of two different input ports of the adder 7.
In the second system, another diverted input signal enters a low pass filter 5. The low pass filter 5 extracts only a low frequency component from the input signal in accordance with predetermined cut-off characteristics. The extracted low frequency component is fed into an overtone-generating unit 4.
The overtone-generating unit 4 generates a signal (an overtone) having a frequency component equal to an integer multiple of a frequency component of the extracted low frequency component. The generated overtone is fed into the adder 7 through the other input port of the adder 7.
The adder 7 adds together the respective signals that have entered the adder 7 through the two different input ports thereof. Results from the addition are fed into an output terminal 2.
There is a variety of methods for generating the overtone. The following discusses, with reference to FIG. 10, a zero-crossing process among the methods.
An overtone-generating example is now contemplated in accordance with a sinusoidal waveform as shown in FIG. 10(a).
A zero-crossing point is a place where a signal switches over between positive and negative values. For example, P1, P2, and P3 in FIG. 10(a) are the zero-crossing points at which a negative signal is turned into a positive one.
To generate a twofold overtone, an original waveform extending from a negative-to-positive zero-crossing point to another, or rather from a distance between P1 to P2 to another between P2 to P3 may be compressed into a half of the original waveform in the direction of a time axis to repeatedly regenerate the compressed waveform twice. As a result, as illustrated in FIG. 10(b), the processed signal has twice as high frequency as that of the original signal.
In general, when “n” is a positive integer, an original waveform extending between the same zero-crossing point is compressed into one over “n” of the original waveform in the direction of the time axis to repeatedly regenerate the compressed waveform a “n”-number of times, thereby generating an “n”-fold overtone.
When complex sound (e.g., a chord or a sound having several frequency components) enters the first prior art acoustic signal-processing apparatus of FIG. 9 (a), then frequency components other than a target overtone to be generated are objectionably produced. As a result, the generated overtone is distorted, with a concomitant degradation in sound quality.
The drawback as discussed above is overcome by a second prior art acoustic signal-processing apparatus of FIG. 9(b). The following discusses the second prior art acoustic signal-processing apparatus with reference to FIG. 9(b). In FIG. 9(b), components similar to those of FIG. 9(a) are identified by the same reference characters.
As illustrated in FIG. 9(b), the second prior art acoustic signal-processing apparatus has improvements in which the complex sound is divided into several frequency bands to generate an overtone based on each component that belongs to corresponding one of the divided frequency bands.
The second prior art acoustic signal-processing apparatus of FIG. 9(b) includes a band-dividing unit 6 that is absent in the first prior art acoustic signal-processing apparatus of FIG. 9(a). The band-dividing unit 6 includes a plurality of band pass filters “5a” to “5c” designed for different frequency bands, thereby permitting a low frequency component in an input signal to be divided into several signals, each of which belongs to corresponding one of the different frequency bands.
The divided signals are fed into overtone-generating units “4a” to “4c”, each of which is provided for a corresponding one of the different frequency bands. In each of the overtone-generating units “4a” to “4c”, an overtone is generated. An adder “7a” adds together output signals from the overtone-generating units “4a” to “4c”. The added output signals are fed into another adder “7b” through one of two different input ports of the adder “7b”. 
In principle, the division of the frequency band as illustrated in FIG. 9(b) generates an overtone based on a single frequency component signal for each of the frequency bands, even when the complex sound enters the second prior art acoustic signal-processing apparatus of FIG. 9(b). This feature suppresses the occurrence of distortional components.
The frequency band-dividing method as discussed above advantageously suppresses degradation in sound quality when the complex sound enters the second prior art acoustic signal-processing apparatus of FIG. 9(b). However, the prior art takes no account of the way in which the overtone should be generated based on the component for each of the divided frequency bands.
The present inventors have revealed based on their studies at this time that a poorly structured overtone degrades tone quality, and results in an insufficient effect on improvements in a feeling of bass sound. Details of those shortcomings are described later. It is understood from the shortcomings that the overtone-generating structure as illustrated in FIG. 9(b) yet remains unsatisfactory.